md: dm-raid should call helper function to clear rdev.

dm-raid currently open-codes the freeing of some members of
and rdev.  It is more maintainable to have it call common code
from md.c which does this for all call-sites.

So remove free_disk_sb to md_rdev_clear, export it, and use it in
dm-raid.c

Signed-off-by: NeilBrown <neilb@suse.de>

diff --cc drivers/md/dm-raid.c
Simple merge

diff --cc drivers/md/md.c
Simple merge

diff --cc drivers/md/md.h
Simple merge